This invention relates generally to electric motors and more particularly to the use of current and temperature responsive devices for deenergizing a motor in the event of fault conditions.
Hermetically sealed refrigerator and air conditioning compressor systems typically mount a compressor and an electrical motor for operating the compressor within a fluid-filled, hermetically sealed container. Lead-through pins extend through a wall of the container in sealed, electrically insulated relation to the wall and to each other to electrically connect the motor in an electrical circuit for operating the compressor. A motor protector is mounted inside the container to be thermally responsive to a rise in temperature of the motor as might result from the occurrence of a fault condition in the motor. Typically, the protector is also connected in the motor circuit in such a way that it is responsive to overcurrent conditions in the motor such as might result from such fault conditions to also interrupt operation of the motor for preventing overheating of the motor.
Motor protectors open the power circuit when either the motor temperature exceeds selected limits or when the motor draws excessive current, either from running overload or locked rotor conditions or a combination of the two conditions. In normal operation, the appliance in which compressors are used rarely experience a protector trip in which the power circuit is opened and the motor protector provides protection against motor failure and a possible fire from an overheated motor for the life of the appliance. Occasionally, due to some abnormal condition, the motor protector, which is automatically resettable, will periodically cycle. The fault condition, if left uncorrected, can eventually cause the motor protector to reach its useful life and fail. Failure is typically manifested by the electric contacts within the protector welding closed due to arcing and subsequent welding. When this happens, the branch circuit protection is relied on to interrupt the supply of power to the appliance. If, for some reason, the branch circuit protection does not open the circuit to the overheated appliance, a danger of fire or other catastrophic condition could occur. Eventual catastrophic failure occurs when the insulation between the motor windings and ground breaks down, due to high heat, and the windings short to ground or turn to turn shorts are formed resulting in a current surge of several hundred or more amperes. This can cause the failure of other components in the current path, such as the glass of a glass sealed, electrical feed through header. As a result, high pressure hot refrigerant gas and lubricating oil could be vented out of the compressor container. In order to prevent this from happening it is known to provide a fusible link placed inside the hermetic motor protector connected in series with the contacts. The link is designed to melt and permanently open the circuit when the current going to the motor exceeds every condition prior to a short circuit to ground, typically the burn-out current is some 25 percent higher than the worst case of normal locked rotor current and is continuous, not intermittent due to the protector cycling.
However, in this approach the thermostatic disc inside the protector must be formed with a higher operating temperature than for comparable thermostatic discs in protectors that do not also contain a fusible link to compensate for the heat generated by the fusible link. The higher stresses required to form a thermostatic snap acting disc with a higher operating temperature can lead to shorter life expectancy of the disc. Due to the higher temperature at which the disc must be calibrated, the thermostatic metal can fatigue and go into creep action with subsequent welding of the contacts. This is particularly true, for example, where momentary power interruptions occur during compressor operation which can result in locked rotor conditions.